He did not address reports that he plans to escalate the US troop level in Iraq but said: "One thing is for certain: I will want to make sure that the mission is clear and specific and can be accomplished."
The US president is set to announce on Friday that he will nominate the US spy chief, John Negroponte, to be deputy secretary of state to Condoleezza Rice, a top aide said.
Bush was to nominate retired vice-admiral Michael McConnell, a former head of the National Security Agency, to replace Negroponte, the aide said on condition of anonymity.
Both positions require approval by the US senate. Bush will name the current US ambassador to Iraq, Zalmay Khalilzad, to the top diplomatic post at the UN, a senior State Department official said.
He would replace John Bolton, whose controversial tenure ended last week.
ABC News reported that the president planned to nominate new leaders for the US Central Command that oversees the wars in Afghanistan and Iraq, as well as the senior military commander in Iraq.
The US president was to nominate the head of US forces in the Pacific, Admiral William Fallon, to replace General John Abizaid at Central Command, and replace George Casey with Lieutenant General David Petraeus, ABC said.
The White House declined to challenge the report.